Known in the art are more than 50 principally different ways for the production of urea, though in commercial processes for the production of urea the following reaction is mainly used: EQU 2NH.sub.3 +CO.sub.2 .revreaction.NH.sub.4 CO.sub.2 NH.sub.2 .revreaction.CO(NH.sub.2).sub.2 +H.sub.2 O
The processes enjoying the most extensive application in the production of urea throughout the world are exemplified by the "Stamicarbon" process (Holland) and the "Mitsui Toatsu" process (Japan).
In the "Stamicarbon" process (cf. French Patent No. 1,184,991; British Patent No. 819,030 or "Nitrogen", 1959, May No. 2, pp. 25-27; Chemical Processing, 1962, v. 25, No. 16 pp. 19-23), liquid ammonia is fed through a pre-heating unit and carbon dioxide, by means of a compressor to a unit for the synthesis of urea. A recycled solution of carbon-ammonium salts (CAS) is pumped into the same unit, the molar ratio between the components in the starting reaction mixture being NH.sub.3 :CO.sub.2 :H.sub.2 O=4.5:1:0.5. The temperature in the mixer is 175.degree. C., in the synthesis column-190.degree. C.; the pressure in the synthesis unit is 200 ata.
The melt from the synthesis column containing 35-38% of NH.sub.3, 10-12% of CO.sub.2, 28 to 35% of CO(NH.sub.2).sub.2 and 19-23% of H.sub.2 O is throttled to 18 ata and delivered to the first step distillation unit consisting of a rectification column, preheater and a separator. In this unit the principal portion (about 90%) of the unreacted ammonia and carbon dioxide is separated from the solution of urea due to reduction of pressure from 200 to 18 ata and heating of the melt to 163.degree. C.
The gases from the first step distillation are fed at a temperature of 120.degree.-125.degree. C. to a washing column, whereinto a solution of CAS is also fed from the second step distillation along with ammonia liquor and liquid ammonia. In this column condensation of water vapor occurs at 92.degree.-96.degree. C. and adsorption of the major amount of carbon dioxide from the distillation gases with the formation of a recycled solution of CAS containing 38 to 45% NH.sub.3, 30 to 37% CO.sub.2 and 22-27% H.sub.2 O. The ammonia vapor purified from carbon dioxide is fed to a condenser, wherefrom a portion of liquid ammonia is fed to spraying in the washing column, while the remaining ammonia is recycled to the synthesis.
The condensed ammonia along with the gases inert to the synthesis of urea is fed to a scrubber sprayed by the liquor steam condensate. The ammonia liquor produced here is used for spraying the washing column and the gas is throttled to atmospheric pressure and vented to the atmosphere through the tailing absorber.
After the first step distillation the synthesis-melt of urea containing 8-11% of NH.sub.3, 1.5 to 2.5% of CO.sub.2, 55-60% of CO(NH.sub.2).sub.2, 28-35% of H.sub.2 O is throttled to a pressure of 2.5-4.0 ata and passed to the second step distillation unit consisting of a rectification column, preheater and a separator.
In this unit the remaining portion of ammonia and carbon dioxide is distilled from the melt at 140.degree.-142.degree. C. The gases resulting from this second step distillation are fed to a condenser-absorber sprayed by the liquor vapor condensate. Here a solution of CAS is formed containing 33-50% NH.sub.3, 10-16% CO.sub.2, 35-55% H.sub.2 O. This solution is then pumped to the washing column.
The unabsorbed gases from the second step distillation unit are fed to an absorber sprayed by the liquor vapor condensate and a solution of ammonia and carbon dioxide circulated through the cooler. Other ammonia-containing gas streams are also fed to this absorber. The absorption heat is removed by means of a cooler. A portion of a weak solution of CAS formed in the absorber is continuously removed through a boiler to a desorber, whereinto live steam is also fed. The temperature in the bottom section of the desorber is maintained at 135.degree.-145.degree. C., pressure is 3 to 4 ata. The gas stream from the desorber (containing 45-60% NH.sub.3, 5-10% CO.sub.2 and 35-45% H.sub.2 O) is fed to the second step distillation condenser-absorber. The cooled water after the desorber is drained to a sewage system.
After the second step distillation the solution (containing 0.8-2.0% NH.sub.3, 0.2-0.5% CO.sub.2, 64-72% CO(NH.sub.2).sub.2, 26-36% H.sub.2 O) is throttled to a residual pressure of 300 mm Hg and fed into a vacuum evaporator for pre-evaporation. In this apparatus the solution temperature is reduced, due to partial evaporation of water, to 95.degree. C., while the content of urea is incrased to 74%. Then a two-step evaporation of the solution is effected. In the first step, the solution is evaporated to 93-95% under a residual pressure of 300 mm Hg and the temperature of 125.degree. C.; in a second step the residual pressure is 20-50 mm Hg and temperature 138.degree. C., whereby the content of urea is increased to 99.7-99.8%. Vacuum is ensured, as a rule, by means of a system of condensers and steam-ejection pumps. The liquor vapor condensate is treated in the above-described system of absorption-desorption.
The melt of urea after evaporation is dispersed into drops and atomized in a granulation tower. Cooling of the resulting granules is effected by air sucked through the granulation tower by ventilators. The granulated product is passed through a classifier (sifter), wherein the nonstandard granules are screened-off, then fed to a dissolving apparatus, wherefrom the resulting solution of urea is delivered to the evaporation stage. The commercial product is delivered to storage or to a consumer.
One of the most essential disadvantages of this prior art process resides in the necessity of using steam at a pressure of about 10 ata as a power stream in vacuum-ejection units, as well as cooling water necessary for its condensation. Furthermore, it should be also noted that a portion of the vapor-gas stream remaining uncondensed after completion of the liquor vapor condensation due to an indirect heat-exchange with the cooling return water contains impurities of ammonia, carbon dioxide and urea which may be present in the gas phase both as vapors and as mist. This vapour-gas mixture is then contacted with the power steam-working stream in an ejector intended for ensuring vacuum in the zone of thicknening of the urea solution. As a result, there contamination of the power steam with ammonia, carbon dioxide and urea occurs. The power steam condensate containing these impurities is added to the liquor vapor condensate and then subjected to separation to purified waste waters removed from the system and a stream of ammonia, carbon dioxide and urea recycled to the process. Due to the power steam condensate the amount of waste waters is increased by 20 to 50%. It is clear that the greater the amount of waste water, the greater the load and rates of power consumption at the purification stage.
Also known in the art is a process for the production of urea belonging to "Mitsui Toatsu", a Japanese company. (Cf. U.S. Pat. No. 3,317,601, British Patent No. 1,047,954, FRG Patent No. 1,299,295 and French Patent No. 1,381,931). This process comprises feeding, into a reactor, gaseous carbon dioxide, liquid ammonia and a recycled solution of CAS and urea. The molar ratio between the components in the starting reaction mixture NH.sub.3 :CO.sub.2 :H.sub.2 O=(3.7-4.5):1:0.4. The synthesis is conducted under a pressure of from 220 to 230 atm and at a temperature within the range of from 180.degree. to 195.degree. C. The temperature control in the reactor is effected by heating of ammonia. The degree of conversion of carbon dioxide to urea amounts to 50-67%.
The synthesis melt of urea from the reactor is fed to a three stage distillation. In the first stage the operation pressure of 18 ata and temperature of 155.degree. C. are maintained, at the second stage--pressure 3 ata and temperature 130.degree., at the third stage pressure is 0.3 ata and temperature is 115.degree. C. The gases from the first stage distillation are admitted to a high-pressure absorber, wherein the formation of a recycled solution of CAS and washing of ammonia vapors from contaminating carbon dioxide are effected. Ammonia is further condensed and a portion thereof is used to spray the absorber, while the major amount of ammonia is recycled to the synthesis. Temperature conditions in the absorber (100.degree. C. in the bottom section and 50.degree. C. in the top section) are ensured by means of a heat-exchanger through which a solution from the vacuum crystallyzer is recycled and due to the supply, to spraying in the absorber, of liquid ammonia and ammonium liquor resulting from washing of the inert gases remaining after condensation of ammonia.
The gases from the second stage of distillation are fed to a low-pressure absorber, wherein water vapors are condensed at a temperature of 50.degree. C. and ammonia and carbon dioxide are absorbed with the formation of a solution of CAS and urea which are then fed to spraying of the high-pressure absorber. The gases from the third stage distillation are washed with the mother liquor obtained after separation of urea crystals in a centrifuge, in a cooler-absorber, wherefrom the solution of CAS and urea is fed to spraying of the low-pressure absorber.
After the third stage of distillation the solution containing 70% urea is fed to a vacuum-crystallizer, wherein evaporation of the solution and crystallization of urea are effected under a residual pressure of 60-70 mm Hg and temperature of about 60.degree. C. The slurry is thickened in a decanter and; the clarified mother liquor is recycled to the vacuum crystallizer by means of a pump through the heat-exchanger of the high-pressure absorber. Vacuum in the crystallization unit is maintained by means of a system of condensers and steam-ejector units.
Urea crystals are partly dehydrated in a centrifuge, dried to a moisture content of 0.2-0.3% and fed to a melting unit positioned over the granulation tower. The molten urea is pulverized across the tower, wherein the granules are air-cooled first to 80.degree.-90.degree. C. and then to a lower temperature in the bottom section of the tower. The granulate is screened and the final product is delivered to storage.
An embodiment of the process for producing urea according to patents to "Mitsui Toatsu" (cf. FRG Patent No. 1,668,856, 1973; French Patent No. 2,099,882, 1972) resides in that said vacuum thickening of the solution of urea is conducted in two steps, namely: by evaporation and crystallization at a temperature below the melting point of urea to form a slurry, where from the residual moisture is removed by heating at a temperature above the melting point.
In order to create a vacuum at the stage of thickening of the solution of urea in both embodiments of the above-discussed process of "Mitsui Toatsu", as well in the "Stamicarbon" process, a system of condensers and steam-ejection units is required, the "Mitsui Toatsu" process possesses the same disadvantages which were mentioned above as characteristic of the "Stamicarbon" process.
It is an object of the present invention to overcome the above-mentioned disadvantages.
It is an object of the present invention to provide a simpler process for the production of urea which would make it possible to lower power consumption.
The object of the present invention is accomplished in a process for producing urea which comprises:
(a) synthesis of urea from ammonia and carbon dioxide under a pressure of from 140 to 400 ata and at a temperature within the range of from 160.degree. to 230.degree. C.; PA0 (b) separation of the resulting aqueous solution of urea from the ammonia and carbon dioxide not converted to the desired product; PA0 (c) separation of liquid or gaseous streams of ammonia and carbon dioxide from the gases which are inert with respect to the synthesis of urea and purified waste water withdrawn from the process; PA0 (d) recirculation of said liquid or gaseous streams of ammonia and carbon dioxide to the stage of the synthesis of the desired product; PA0 (e) vacuum-thickening the resulting solution of urea under a pressure of from 0.04 to 0.90 ata with the recovery of dehydrated urea in the condensed phase and a mixture of water, ammonia and carbon dioxide in the vapor phase; PA0 (f) converting the dehydrated urea into solid particles and cooling in a current of air; PA0 (g) water washing urea dust from the air from the stage of converting the dehydrated urea into solid particles and their cooling to give an aqueous solution of urea recycled to said stage of vacuum thickening; PA0 (h) condensation of the mixture of water, ammonia, carbon dioxide and urea from the vapor phase resulting from the stage of vacuum thickening due to its indirect cooling with the formation of a liquor vapor condensate; PA0 (i) supplying the condensate of the liquor vapor to the stage of recovery to gases inert with respect of the synthesis of urea and purified waste water; PA0 (j) supplying ammonia and carbon dioxide contained in the uncondensed portion of the vapor phase from the stage of vacuum thickening after said indirect cooling to the stage of separation of the gases inert with respect to the synthesis of urea and purified waste water, wherein in accordance with the present invention the non-condensed portion of the vapor phase from the stage of vacuum thickening after said indirect cooling is subjected to absorption condensation under a pressure at which said vapor phase is delivered to the stage of absorption-condensation, said absorption-condensation being effected by way of direct contact of said vapor phase with a cooling agent-absorbent having a temperature within the range of from 10.degree. to 60.degree. C., pressure of from 1.5 to 18 ata and containing dissolved in water 0.2 to 3.4% by weight of ammonia, 0 to 1.0% by weight of carbon dioxide and 0 to 50% by weight of urea, whereafter at least a portion of the resulting solution is fed to the stage of separation of the gases inert with respect to the synthesis of urea and purified waste water or to the stage of separating of the aqueous solution of urea from the ammonia and carbon dioxide not converted to the desired product, or to the stage of vacuum thickening of the solution of urea.
An embodiment of the process according to the present invention is that said adsorption-condensation is effected using said liquor vapor condensate as a cooling agent-absorbent.
Another embodiment of the present invention is that as the cooling agent-absorbent, use is made of waste waters purified from contaminating urea under a pressure of from 16 to 18 ata in the stage of separation of the gases which are inert with respect to the synthesis of urea and purified waste water.
Still another embodiment of the process according to the present invention is that as the cooling agent-absorbent, use is made of a solution of urea resulting from water washing of urea dust from the air from the stage of converting the dehydrated urea into solid particles and cooling, and/or a solution of urea resulting from separating of the aqueous solution of urea from the ammonia and carbon dioxide not converted to the desired product.
The process according to the present invention makes it possible to avoid supplying power steam to the zone of contacting with the vapor-gas stream remaining after completion of the liquor vapor condensation due to an indirect cooling. As a result, the rate of consumption of steam and return water (necessary for steam condensation) is reduced. Furthermore, owing to contacting, in the absorber-condenser, of the vapor-gas stream (remaining after completion of the liquor vapour condensation by indirect cooling) with the aqueous absorbent (for example, with the cooled liquor vapor condensate) there occurs a substantially complete absorption-condensation of ammonia, carbon dioxide and urea contained in said vapor-gas stream. In doing so, in contrast to the prior art processes, as a result of recuperation of said impurities there is no any additional formation of waste waters; i.e. the total amount of waste waters from the plant of urea production is reduced. Due to the lowered amount of waste waters, the power consumption for their treatment is reduced, the degree of purification is increased along with reduced losses of the starting materials and the desired product.
Vacuum in the zone of thickening of the solution of urea is maintained, according to the present invention, by absorption-condensation of the vapor-gas stream remaining after completion of the liquor vapor condensation by an indirect cooling upon contacting with the aqueous absorbent, as well as due to utilization of the potential energy of the aqueous absorbent stream taken-off at the point of the process flowsheet, where the pressure of this stream is above atmospheric. When it is required to increase the pressure of the aqueous absorbent stream by 2-3 ata by means of a pump (taking into account the fact that liquids are substantially non-compressible media), the electric power consumption (for operation of the pump motor) is rather low and costs of electric power are generally smaller than those of steam.
The practical application of the process according to the present invention eventually makes it possible to lower the steam consumption rate (per ton of urea) by 0.1-0.25 ton/ton and the consumption rate of return water-by 6-14 m.sup.3 /ton.